Should ClassLoader::getResouces return the same resource twice?

[Johannes Spangenberg]

> Looks like the ModuleLayer.defineModules is the thing I needed.
> With it, I can use my current classloader to load all the modules
> and it can find the classes from both module and unnamed module.

Note that class loaders for modules behave differently then original 
class loaders. For example they will favor local class definitions over 
class definitions of the parent loader. Not sure if there are other 
important differences. The loader is implemented in 
jdk.internal.loader.Loader [1].

> There is some trick to get a class from an upper layer to bind to the
> a class in the boot layer?

You can pass the created module layer as first argument to 
ServiceLoader.load​(ModuleLayer, Class) [2].

[1] 
https://github.com/openjdk/jdk/blob/master/src/java.base/share/classes/jdk/internal/loader/Loader.java

[2] 
https://docs.oracle.com/en/java/javase/15/docs/api/java.base/java/util/ServiceLoader.html#load(java.lang.ModuleLayer,java.lang.Class)